Weak materials are ones that don't take a reference on their parent and
they are associated with a callback that notifies when the material is
destroyed, because its parent was freed or modified.
More details can be found at:
http://wiki.clutter-project.org/wiki/CoglDesign/CoglMaterial
For now the concept is internal only but the plan is to make this public
at some point once we have tested the design internally.